Not applicable.
1. Field of the Invention
This invention relates to a process for the removal of organic sulfur compounds by oxidation from hydrocarbon fuels which have relatively low amounts of sulfur present.
2. Background of the Invention
The presence of sulfur in hydrocarbons has long been a significant problem from the exploration, production, transportation, and refining all the way to the consumption of hydrocarbons as a fuel. As government regulations throughout the world increasingly restrict sulfur levels in fuels, the problem of sulfur reduction is being felt by producers, refiners, transporters and marketers of the full range of fuel products, from gasoline and diesel fuel to jet fuel, kerosene, heating oil and heavier fuels. In Western Europe, North America, Japan and other industrial nations the sulfur restrictions on gasoline and on-highway diesel fuel are moving to the ultra-low levels of 30, 15 or even 10 ppm. Consequently, producers, refiners and marketers are seeking low-cost technologies for producing ultra-low sulfur products, with maximum use of existing facilities.
Sulfur can be found in almost any stream within a refinery complex. This is due to the sulfur in the crude oil to be processed. The process technology for removing sulfur that is in almost universal use today is hydrotreating, sometimes referred to as hydrodesulfurization. Hydrotreating, as used herein, is a process whose primary purpose is to reduce the sulfur and/or nitrogen content (and not to change the boiling range) of the feed. While there are many variations and improvements, this technology requires high temperature and pressure in a hydrogen environment and employs advanced catalysts. This process successfully removes the majority of the sulfur compounds in hydrocarbons. However, the substituted thiophenes, especially those having steric hindrance of the sulfur, are particularly difficult and require high severity hydrotreaters having pressures well in excess of 500 psi. Achieving ultra-low sulfur levels requires that most of these difficult-to-hydrotreat compounds be removed, which could drive many refiners to install new hydrotreaters or carry out expensive revamps of their existing hydrotreaters. Gasoline fractions will contain mainly thiophenes and some benzothiophenes but generally little or no dibenzothiophenes. Diesel fractions will contain mainly benzothiophenes and dibenzothiophenes.
The prior art is replete with attempts to reduce the sulfur content of hydrocarbon by both reduction and oxidation of organic sulfur present. Much of this prior art relating to oxidation has taught the use of various peroxides in conjunction with a carboxylic acid and, specifically, the preferred species involved in the practice of this invention; i.e., hydrogen peroxide and formic acid. For example, U.S. Pat. No. 5,310,479 teaches the use of formic acid and hydrogen peroxide to oxidize sulfur compounds in crude oil, limiting the application of the technology only to aliphatic sulfur compounds. There was no hint of the removal of aromatic sulfur compounds. This patent discussion is directed to the removal of sulfur from crude oil rich (about 1-4%) in sulfur compounds. The acid to peroxide ratio was indiscriminately broad and failed to recognize the economic disadvantages to using hydrogen peroxide in attempts to remove large amounts of sulfur, while at the same time failing to recognize the importance of controlling the presence of water to the successful operation. Water was used to extract the sulfones from the treated hydrocarbon in a separate wash step. Further, the prior art also fails to recognize the beneficial effect of limiting the peroxide concentration to low values without compromising either the rate or extent of oxidation of the sulfur compounds.
A recent study entitled xe2x80x9cOxidated Desulfurization of Oils by Hydrogen Peroxide and Heteropolyanion Catalyst,xe2x80x9d Collins, et al., published Journal of molecular Catalysis A: Chemical, 117 (1997) 397-403, discusses other studies to oxidatively remove sulfur from fuel oil, but large quantities of hydrogen peroxide were required. However, the experimental work did show that unacceptable amounts of hydrogen peroxide were consumed thus suggesting the cost of oxidative reduction of sulfur in feedstocks for diesel fuel to be impermissibly high.
In European Patent Application Publication No. 0565324A1, a method for recovering organic sulfur compounds from liquid oil is described. While the stated objective of the patent publication is to recover the organic sulfur compounds, the treatment involves using a mixture of a number of oxidants, one of which is disclosed as a mixture of formic acid and peroxide. The distillation products, the organic sulfones, are removed by a number of methods including absorption on alumina or silica adsorbent materials. The treatments described are characterized by use of a low ratio of formic acid to the hydrogen peroxide.
While this and other prior art recognize the reaction kinetics and mechanism of hydrogen peroxide and other peroxides with organic sulfur compounds present in various fuels, none recognize the combination of factors necessary to successfully and economically remove relatively small amounts of sulfur present in fuels such as diesel oil, kerosine, gasoline, and light oils down to residual levels approaching zero. While low amounts of sulfur will be construed to mean in the context of this invention, those amounts which are less than about 1500 parts per million, an example demonstrates effective removal of 7000 ppm of sulfur such that the present invention is applicable to higher levels of sulfur. Of course in some instances, the practice of this invention may be economically and technically applicable to the treatment of fuels having a sulfur content at these elevated levels. It has been found in the practice of this invention that the sulfur content of the fuel which is left unoxidized is less than about 10 ppm of sulfur, often as low as about 2 ppm to about 8 ppm. Oxidation alone does not necessarily ensure total removal of the sulfur to the same low residual sulfur values since some of the oxidized sulfur species do have a non-zero solubility in the fuel, and a partition coefficient that defines their distribution in the oil phase in contact with a substantially immiscible solvent phase, whether it is an organic solvent as in prior art, or the high acid aqueous phase of this invention. In addition to the substantially complete and rapid oxidation of the relatively low amounts of sulfur in the fuel feed, the present invention also teaches the substantially complete removal of the oxidized sulfur to residual levels approaching zero, and the recovery of the oxidized sulfur compounds in a form suitable for their practical further disposition in an environmentally benign way. Removal of sulfur by oxidation adds another dimension in that it is desirable that such removal of sulfur be accomplished, without reducing the octane rating of the gasoline, whether it be motor, racing or aviation gasoline. This object is accomplished by the present invention.
In an article, Desulfurization by Selective Oxidation and Extraction of Sulfur-Containing Compounds to Economically Achieve Ultra-Low Proposed Diesel Fuel Sulfur Requirements (Chapados, et al., NPRA Presentation, Mar. 26-28, 2000) the oxidation step involved the reaction of the sulfur in a model compound using dibenzothiophene with a peroxyacetic acid catalyst made from acetic acid and hydrogen peroxide. The reaction with the peroxyacid was conducted at less than 100xc2x0 C. at atmospheric pressure and in less than 25 minutes. After extraction, the process resulted in a reduction of the sulfur content in the diesel fuel. Still, the cost was indicated to be high with the hydrogen peroxide being the biggest cost item and consumed in the process due in large part to the lack of recognition of the part excessive water plays in the efficient utilization of low amounts of hydrogen peroxide.
It has been discovered that fuel products, such as gasoline and diesel fuel, though meeting the present requirements of about 500 ppm maximum sulfur content, can be economically treated to reduce the sulfur content to an amount of from about 5 to about 15 ppm, in some instances even less and not diminish the octane rating of the fuel product. In practicing the process of the present invention the hydrocarbon fuel containing low amounts of organic sulfur compounds, i.e., up to about 1500 ppm, is treated by contacting the sulfur-containing fuel with an oxidizing solution containing hydrogen peroxide, formic acid, and a limit of a maximum of about 25 percent water. The amount of the hydrogen peroxide in the oxidizing solution is greater than about two times the stochiometric amount of peroxide necessary to react with the sulfur in the hydrocarbon fuel. The oxidizing solution used contains hydrogen peroxide at low concentration, the concentration, in its broadest sense, being from about 0.5 wt % to about 4 wt %. The reaction is carried out at a temperature ranging from about 50xc2x0 C. to about 130xc2x0 C., a pressure ranging from about 15 psi to about 250 psi, for less than about 15 minutes contact time at optimum conditions. The elevated pressure is to due to the volatility of the feed stream. The total pressure of the reaction will be dependent upon the partial vapor pressures of hydrocarbon, formic acid, hydrogen peroxide, and water at the selected operating temperature, plus additional pressure from any additioal inert gas. The partial vapor pressure contribution from a naphtha at 95xc2x0 C. will be about 25 psi. The partial vapor pressure contribution of a diesel at 95xc2x0 C. will be nil. The oxidizing solution of the invention has, not only a low amount of water, but small amounts of hydrogen peroxide with the formic acid being the largest constituent. Some of the oxidation products, usually the corresponding organic sulfones, become soluble in the oxidizing solution and, therefore, may be removed from the oxidized fuel by a subsequent phase separation step. The aqueous phase is removed from the hydrocarbon phase now having a reduced sulfur content. Depending on the solubility of the sulfones, some sulfones may be left in the hydrocarbon phase. Where there is a residual amount of oxidized sulfur compounds, usually sulfones, in the hydrocarbon fuel, this invention enables the practical and economic use of additional separation steps to remove the sulfones from the hydrocarbon fuel. Sulfones may be removed by selected solid adsorbants such as, for example, in a cyclic adsorption-desorption operation to achieve a substantially sulfur-free hydrocarbon fuel product. The adsorption-desorption operation also recovers the oxidized sulfur compounds in a concentrated form, practical for their final, environmentally benign, disposition within a refinery. It is another object of the invention that the fuel goes through this oxidation process while leaving undisturbed the carbon to hydrogen ratio and the olefininc compounds which contribute to the octane rating of the hydrocarbon fuel. Octane rating is the resistance of the hydrocarbon fuel to burn. The higher the rating, the slower the burn when ignited during the compression burn cycle of the piston. Higher octane allows for better control of burning for high compression engines.
Once the extract containing the oxidized sulfur compounds is separated from the desulfurized hydrocarbon fuel, or raffinate, the extract can be treated to recover the acid for recycle. The separation between the hydrocarbon phase and the aqueous phase can be accomplished in a number of ways, but the preferred separation occurs by the use of a liquid-liquid separator operated at a temperature sufficiently high, close to the oxidation reaction temperature, to result in gravity separation of the material without the appearance of a third, precipitated solid phase. The aqueous phase, of course, being heavier than the hydrocarbon phase would be drained from the bottom of the separation device where it may be preferably mixed with a suitable high boiling range refinery stream, such as for example, a light gas oil,and flash distilled to remove the water and acid overhead while transferring and leaving the sulfur-containing compounds in the gasoil stream exiting at the bottom of the distillation column. The overhead stream containing acid and water from the flash distillation tower is further distilled in a separate column to remove a portion of its water for disposal. The acid recovered can then be returned to the oxidizing solution make up tank where it is combined with the hydrogen peroxide to form the oxidizing solution and again contact the sulfur-containing fuel feed. This preservation of the acid enhances the economics of the process of this invention.
After separation, the hydrocarbon fuel may be contacted with a caustic solution, or with anhydrous calcium oxide (i.e., quicklime) and/or passed through filtering devices to neutralize any trace acid remaining and to make a final dehydration of the hydrocarbon fuel. The hydrocarbon fuel may then be passed over a solid alumina bed, at ambient temperature, to adsorb the residual oxidized sulfur compounds soluble in the hydrocarbon fuel, if any are present. The hydrocarbon fuel product is now substantially desulfurized, neutralized, and dry.
The oxidized sulfur compounds adsorbed on alumina may be removed by desorption and solubilization into a suitable hot polar solvent, methanol being the preferred solvent. Other suitable solvents are acetone, THF (tetrahydrofuran), acetonitrile, chlorinated solvents such as methylene chloride as well as the aqueous oxidizer solution with high acid contents of this invention. One advantage of the adsorption/desorption system of this invention is that it can use commericallyavailable alumina adsorbants that are used in multiple cycles without significant loss of activity and without the need to reactivate them by conventionally employed high temperature treatment for dehydration. The extracted oxidized sulfur compounds are transferred into higher boiling refinery streams for further disposition by flash distillation, which also recovers the methanol for recycle in the alumina desorption operation.
The oxidizing solution of the invention is preferably formed by mixing a commercially-available 96 wt %, by weight, formic acid solution with a commercially-available hydrogen peroxide solution normally the 30 wt %, 35 wt % and 50 wt % concentration commercially available in order to avoid the dangers connected with handling a 70 wt % hydrogen peroxide solution in a refinery environment. The solutions are mixed to result in an oxidizing material containing from about 0.5 to about 4 wt % hydrogen peroxide, less than 25 wt % water with the balance being formic acid. The water in the oxidizer/extractor solution normally comes from two sources, the dilution water in the peroxide and acid solutions used, and the water in the recycled formic acid which includes part of the water formed in the oxidation reaction, when the process operates in the recycle mode. On occasion, additional water could be added without being detrimental to the practice of this invention as long as the criteria explained herein are considered, but it is important to an economical process to keep the water content low as set forth herein. The preferable concentration of hydrogen peroxide, which is consumed in the reaction, in the oxidizer solution would be from about 1% to about 3% by weight, and most preferably from 2 to 3 wt %. The water content would be limited to less than about 25 wt %, but preferably between about 8 and about 20%, and most preferably from about 8 to about 14 wt %. The oxidation/extraction solution used in the practice of this invention will contain from about 75 wt % to about 92 wt % of carboxylic acid, preferably formic acid, and preferably 79 wt % to about 89 % wt formic acid. The molar ratio of acid, preferably formic acid, to hydrogen peroxide useful in the practice of this invention is at least about 11 to 1 and from about 12 to 1 to about 70 to 1 in the broad sense, preferably from about 20 to 1 to about 60 to 1.
This will accomplish a rapid and complete oxidation of the sulfur compounds, and their substantial extraction from such refined products as diesel fuel, jet fuel, or gasoline, whether motor, racing or aviation gasoline, which contain from about 30 to about 1500 ppm sulfur and will perform effectively to oxidize and extract organic sulfur present in fuels at greater concentrations. Since the moles of hydrogen peroxide to be used is proportional to the amount of sulfur present and since the peroxide is consumed, the cost of this material can have a negative effect on the economics of the operation if the amount of sulfur present is high. Therefore, this invention is properly most useful for polishing small amounts of sulfur, such as for example less than about 1500 ppm, from hydrocarbon fuels ready for market than for removal of sulfur from crude oil containing gross amounts of sulfur.
In the oxidation of organic sulfur compounds using hydrogen peroxide, the stochiometric reaction ratio is two moles of the hydrogen peroxide consumed per mole of sulfur reacted. In the practice of this invention the amount of oxidizing solution used should be such that it contains at least about two times the stochiometric amount to react the sulfur present in the fuel, preferably from about two to about four times. Greater amounts could be used, but only at increased cost since it has been found that improvement of sulfur oxidation is marginal at best when the amount is greater than four times the amount needed. Furthermore, to minimize peroxide losses by decomposition side reactions, the hydrogen peroxide concentrations in the oxidizer composition of this invention are preferably adjusted at low levels about 0.5 wt % to about 4 wt %. At these levels and the reaction temperature of about 95xc2x0 C. and a pressure of about 15 to about 250 psi, depending upon the hydrocarbon involvedit has been discovered that the rapid and complete oxidation, and extraction, of the sulfur compounds from hydrocarbon feeds of relatively low sulfur content, compete favorably with the side reaction of peroxide decomposition, resulting in a practical and economic process for desulfurization of such fuels.
By limiting the water and hydrogen peroxide present and the reaction conditions of this invention, a practical process results with almost complete oxidation of organic sulfur compounds at high rates, with low peroxide concentrations, at relatively small peroxide excess over the stoichiometric requirement, and on feeds with relatively low sulfur content; all of these conditions being recognized in the art as kinetically unfavorable conditions. In addition to this unexpected result, it is accomplished with little loss of the expensive hydrogen peroxide to expected side reactions of self decomposition, or with other hydrocarbon species.
While the following invention is described in some detail, it must be understood by those skilled in the art that there is no intention on the part of the inventors hereof to abandon any part of the concepts of this invention with respect to the reduction of the organic sulfur in fuels and light oils.